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ABOUT THIS IMAGE:

A nearly perfect ring of hot, blue stars pinwheels about the yellow
nucleus of an unusual galaxy known as Hoag's Object. This image
from NASA's Hubble Space Telescope captures a face-on view of
the galaxy's ring of stars, revealing more detail than any existing
photo of this object. The image may help astronomers unravel clues
on how such strange objects form.

The entire galaxy is about 120,000 light-years wide, which is
slightly larger than our Milky Way Galaxy. The blue ring, which is
dominated by clusters of young, massive stars, contrasts sharply
with the yellow nucleus of mostly older stars. What appears to be
a "gap" separating the two stellar populations may actually contain
some star clusters that are almost too faint to see. Curiously, an
object that bears an uncanny resemblance to Hoag's Object can be
seen in the gap at the one o'clock position. The object is probably
a background ring galaxy.

Ring-shaped galaxies can form in several different ways. One
possible scenario is through a collision with another galaxy.
Sometimes the second galaxy speeds through the first, leaving a
"splash" of star formation. But in Hoag's Object there is no sign
of the second galaxy, which leads to the suspicion that the blue
ring of stars may be the shredded remains of a galaxy that passed
nearby. Some astronomers estimate that the encounter occurred about
2 to 3 billion years ago.

This unusual galaxy was discovered in 1950 by astronomer Art Hoag.
Hoag thought the smoke-ring-like object resembled a planetary
nebula, the glowing remains of a Sun-like star. But he quickly
discounted that possibility, suggesting that the mysterious object
was most likely a galaxy. Observations in the 1970s confirmed this
prediction, though many of the details of Hoag's galaxy remain a
mystery.

The galaxy is 600 million light-years away in the constellation
Serpens. The Wide Field and Planetary Camera 2 took this image
on July 9, 2001.